This invention relates to a control arrangement adaptable to an electronic switching system comprising computers used as control units (C.U.).
The use of high-speed data processing circuits has brought about an increase (as compared to non-electronic switching systems) in the centralization of control functions. This centralization has many advantages, but it can make the system vulnerable to component failures if not properly handled. For example if no redundancy were provided it would be possible for a single component failure to cause a complete failure of the system. A duplication of central control computers is therefore necessary.
In view of what has been achieved in the field of telephonic or telegraphic switching, where reliability of operation is of particular significance, a distinction is drawn between arrangements with two control units and arrangements with multiple control units, multiple in this context meaning that the number of control units is greater than two.
Generally speaking, there are two main categories of control arrangements with two control units. In the first category, the two units normally work in parallel, each performing all the current functions, but for only half of the incoming signals. In the event of failure, one of the units handles all the traffic it can, but this presupposes a significant reconfiguration of the network because that part of the content of each store (memory) which is governed by the processed signals differs for the two stores.
In the second category, the two units work in microsynchronism, the second unit repeating the operations of the first, enabling them to be verified, and replacing the other unit in the event of failure. The disadvantage is the waste of the total processing capacity used.
In either category, the concentration of the intelligence and of the memory in only two control units makes the arrangement as a whole inflexible to a large extent and also makes it impossible rationally to adapt standardized control units to telephone or telegraph exchanges of different capacity.
Although it is of course possible to use control units comprising stores differing in capacity according to requirements, the other elements of the control units designed for the maximum capacity of traffic would become plethoric for a low density of traffic.
A control arrangement with multiple control units working in parallel for the various input signals, each operating for (1/n).sup.th of the traffic, have also been proposed. As in the two categories of arrangements comprising two control units, each of the n control units performs all the functions. In this way the control arrangement can be better adapted to the requirements of the network because it is sufficient to vary the number n.
However, at the risk of an enormous waste of memory capacity in regard to the permanent or semipermanent data relating to the nature of the operations to be carried out, there can be no question of providing each of the n control units with its own store supplying it with all the necessary data.
In the known arrangement, the difficulty has been obviated by giving the n control units a common store.
However, the principle of safety becomes illusory when the failure affects precisely the common store which then has to be doubled.
On the other hand, multiprocessing in one and the same store or in two identical stores gives rise to problems of logic (time sharing) and hardware (changing priority access logic) which increase in proportion as the number of control units varies.
The present invention relates to a control arrangement by which these various disadvantages may be obviated.